[Xakepa]

Check your grounds; shorten the OPAG, it's easy with the sockets, the offset should go away. I'm having that same setup (8620/10, 2-1-2 BUF634P) and it's ... excellent.

Opamps might be osciallating, but it seems unprobable - 8620/10 are very stable, you have STEPS, and you measure low quiescent

If you want to know what is a really hot chip, try AD843...first couple of hours it smells

Enjoy

 

[Xerophase]

Quote:

Originally Posted by The Monkey ...And the chips are just short of burning my fingers. I don't know.

Realizing this is several weeks old and you've probably already answered your question by now...

Since you're expecting about 10ma through each buffer it sounds like you're using R11. I've built several PIMETAs, and when I stack them in wide bandwidth mode, they definitely get hot. I went through all the steps you did, and everything looked fine. I finally came to this understanding... your top buffer is trying to dissipate its heat while still acting as a heatsink for the one below it. If you compare this to the 13ma the AD843 draws, which is universally understood to be a hot chip, this all makes sense. In my setup, I've compared the heat on an 843 to the stacked 634s, and the 634s were nearly as warm.

According to the datasheet for the 634Ps they can take up to 125C so if you're "just short of" burning yourself, you're probably OK.

That said, I got paranoid enough to fix some heatsinks on mine. They run fairly cool now and I've put hundreds of hours on my amp with no problems.

Good luck!

 

[The Monkey]

Thanks Xerophase! I actually have not fixed this problem yet, but what you say makes sense to me. Unfortunately, I also have this weird intermittent offset problem.

I think my Pimeta is haunted.

 

[NeilR]

Quote:

Originally Posted by The Monkey Thanks Xerophase! I actually have not fixed this problem yet, but what you say makes sense to me. Unfortunately, I also have this weird intermittent offset problem. I think my Pimeta is haunted.

Have you built an alter to The God of Good Builds?

Do you sacrafice an occasional virgin at that alter?

Now do you inderstand your problem?

If that doesn't work, try reflowing all the joints in the affected channel (intermittant offset). That exorcizes a lot of demons and will work in a pinch if you can't scrounge up a virgin.

 

[Xerophase]

Quote:

Originally Posted by The Monkey Unfortunately, I also have this weird intermittent offset problem.

I'm not sure how much offset you're getting with the AD86xx, but if it's REALLY high maybe the chips are bad? If you have any others you might try throwing them in.

Also, have you measured the voltage your power supply is putting out to verify it's running at 24V? I have no idea what affect too much voltage would have on offset, but at 24V you're pretty close to the 86xx max...

 

[mono]

You have wierd DC offset but no coupling caps on the signal input. Have you confirmed that the source has no offset? Have you tried another source? You might also temporarily add caps in series on the input to the amp to see what happens.

 

[n_maher]

Quote:

Originally Posted by mono You have wierd DC offset but no coupling caps on the signal input. Have you confirmed that the source has no offset? Have you tried another source? You might also temporarily add caps in series on the input to the amp to see what happens.

The Monkey and I looked at his amp at the National Meet and he does in fact have the weirdest behaving amp I've come across (that I didn't build). The offset in both channels would vary from 0 to 50mV (or more) in a matter of seconds. It was somewhat cyclical in its behavior but for the life of me I couldn't come up with a good reason for it.

 

[The Monkey]

Quote:

Originally Posted by n_maher The Monkey and I looked at his amp at the National Meet and he does in fact have the weirdest behaving amp I've come across (that I didn't build). The offset in both channels would vary from 0 to 50mV (or more) in a matter of seconds. It was somewhat cyclical in its behavior but for the life of me I couldn't come up with a good reason for it.

See guys? Haunted!

I'm going to open it up this weekend (work permitting

) and check everything very carefully. As we all know too well, there likely is something small and stupid that I just haven't noticed.

 

[Xerophase]

Quote:

Originally Posted by The Monkey See guys? Haunted!

I'm still hung up on the chip thing... one works fine, the other has crazy problems. If they're not bad, maybe you could post some pictures of your 86xx's mounted to the adapters? They're sometimes easy to get on backwards. I've had some that didn't even have pin 1 marked...

 

[Jazper]

thoughts:

first try reflowing the solder joints ..

second check the temperatures of the various resistors (none should be hot to touch)

third check that you have the right resistor values throughout

fourth try replacing C1 on both the left and right channels

 

[The Monkey]

Quote:

Originally Posted by Jazper thoughts: first try reflowing the solder joints .. second check the temperatures of the various resistors (none should be hot to touch) third check that you have the right resistor values throughour fourth try replacing C1 on both the left and right channels

I've checked most of the joints and they're ok (I reflowed a couple during the build), but can't hurt to check again. The resistors are not getting hot. And I'd really like to avoid C1, but maybe I'll just put it in temporarily. Right now, main suspicion is that I did something weird with hooking up my Tread and switched jack. And, of course, the chips.

 

[Jazper]

Quote:

Originally Posted by The Monkey I've checked most of the joints and they're ok (I reflowed a couple during the build), but can't hurt to check again. The resistors are not getting hot. And I'd really like to avoid C1, but maybe I'll just put it in temporarily. Right now, main suspicion is that I did something weird with hooking up my Tread and switched jack. And, of course, the chips.

You could post pics of the tread...

 

[nikongod]

does the problem change when you use diferent opamps/buffers?

just swap your l&r buffers if you dont have spares.

 

[The Monkey]

Quote:

Originally Posted by Jazper You could post pics of the tread...

Yup, I'll do that when I have a chance.